The bioavailability of a pharmaceutical drug depends largely in part on the solubility and stability of the drug. Many methods have been employed to improve bioavailability of a drug, including, but not limited to, pH adjustment, associating the drug in micelles of detergents, solubilization in an organic solvent, complexation with cyclodextrin or other polymers, and encapsulating the drug in a liposome bilayer (Strickley, R. G., Pharmaceutical Research, No. 21, 2004: 201-230). Either the drug itself or the excipients used to solubilize the drug may have side effects such as allergic reaction or hemolysis.
It is known that the solvents (e.g., ethanol, propylene glycol, polyethylene glycol, dimethylacetamide, dimethylsulfoxide (“DMSO”)), complexing agents (for example, nicotinamide), and surfactants (for example, sodium oleate) are hemolytic and are therefore undesirable for use in injectable solutions. Other limitations to using organic solvents in injectable products include precipitation, pain, and inflammation upon injection.
Liposomes are microscopic lipid vesicles that are composed of a central aqueous cavity surrounded by a lipid membrane formed by concentric bilayer(s) (lamellas). Liposomes are able to incorporate hydrophilic substances (in the aqueous interior) or hydrophobic substances (in the lipid membrane). Liposomes can be unilamellar vesicles (“UMV”), having a single lipid bilayer, or multilamellar vesicles (“MLV”), having a series of lipid bilayers (also referred to as “oligolamellar vesicles”). The multilamellar vesicles typically range in size from 0.2 μm to 10 μm in diameter. See e.g., WO 98/006882. Although anti-hemolytic measures are commonly taken in formulations, maintaining a sufficient amount of liposome in formulation may not be feasible due to the incompatibility of the liposome with an excipient, or the instability of the liposome in the formulation. Further, reconstituting lyophilized formulations containing hydrophobic drugs is often difficult. Moreover, liposomes are not stable in formulations containing concentrated organic solvents.
Unilamellar vesicles with a diameter of less than 0.2 μm (e.g. between 0.02 and 0.2 μm) are commonly known as small unilamellar vesicles (“SUV”). Unilamellar vesicles with a diameter greater than 0.45 μm (in some cases greater than 1 μm) are commonly known as large unilamellar vesicles (“LUV”).
The bilayer(s) of liposomes most often comprise phospholipids, but may also comprise lipids including but not limited to fatty acids, fatty acid salts and/or fatty alcohols. The properties of the liposomes depend, among other factors, on the nature of the constituents. Consequently, if liposomes with certain characteristics are to be obtained, the charge of its polar group and/or the length and the degree of saturation of its fatty acid chains must be taken into account.
In addition, the properties of liposomes may be modified, e.g., to incorporate cholesterol and other lipids into the membrane, change the number of lipidic bilayers, or covalently join natural molecules (e.g., proteins, polysaccharides, glycolipids, antibodies, enzymes) or synthetic molecules (e.g., polyethyl glycol) to the surface. There are numerous combinations of phospholipids, optionally with other lipids or cholesterol, in an aqueous medium to obtain liposomes. Depending on the method of preparation and the lipids used, it is possible to obtain vesicles of different sizes, structures, and properties.
Another important parameter to consider with respect to the formation of liposomes is the rigidity of the lipidic bilayer. The hydrated lipid that focus part of the bilayer may be in either a liquid-crystalline (fluid) or gel state. As the temperature increases, the gel state is converted into the liquid-crystalline state. This occurs at a temperature known as the transition temperature (Tc), which is specific to each lipid. The Tc is directly proportional to chain length and inversely proportional to the degree of unsaturation of the fatty acids and depends on the nature of the polar group.
Despite this, common methods in the preparation of lipid vesicles, such as liposomes, comprise evaporating an organic solvent in which the lipids are dissolved and then dispersed in an optionally buffered aqueous solution. One exemplary method, known as the Bangham method, was originally described in Bangham et al., J. Mol. Biol., 11:238-252 (1965). Variations of the Bangham method are known by those skilled in the art, some of which are described below.
Hydration of a thin lipidic layer. Starting with the organic solution of the constituent lipids of the bilayer, a lipidic film is prepared through removal of organic solvent, which can be achieved by means of evaporation (e.g., at reduced pressure in a rotary evaporator) or by lyophilization. The dry lipidic film obtained is hydrated by adding an aqueous solution and agitating the mixture at temperatures above the Tc.
Reverse-phase evaporation. Starting with the organic solution of the constituent lipids of the bilayer, a lipidic film is prepared through removal of the organic solvent. The system is purged with nitrogen and the lipids are re-dissolved in a second organic solution, usually constituted by diethyl ether and/or isopropyl ether. The aqueous phase is added to the re-dissolved lipids. The system is maintained under continuous nitrogen. A gel is formed by removing the second organic solvent.
Solvent injection. The lipids, dissolved in an organic solvent, are injected slowly into an aqueous solution. The organic solvent used is often a water-miscible solvent, and the aqueous solution may be warmed.
Additional methods for the preparation of multilamellar vesicles can be found, e.g., in Szoka and Papandjopoulos, Ann. Rev. Biophys. Bioeng., 2: 467-508 (1980), and Dousset and Douste-Blazy, Les Liposomes, Puisieux and Delattre, Editors, Tecniques et Documentation Lavoisier, Paris, pp. 1-73 (1985).
Further, when the incorporation of more than one lipid is desired, the lipids should remain homogeneously distributed in the liposomal vesicles. Traditionally, this is achieved by previously dissolving the lipids in an organic solvent and using the resulting organic solvent for preparing the liposomes.
U.S. Pat. No. 4,508,703 describes a method for obtaining powdery mixtures of at least one amphyphilic lipid and, optionally, at least one component of a hydrophobic or partially hydrophobic nature, a method which includes dissolving the components of the mixture in at least one organic solvent and atomizing the obtained solution into an inert gas. The method permits the preparation of lipidic mixtures which can be easily dispersed in an aqueous medium but does not avoid the use of organic solvents.
WO 92/10166 describes a method for preparing liposomes with an elevated encapsulation capacity. The method permits the use of mixtures of lipids; however, the mixture is obtained by means of previous dissolution of the lipids in an organic solvent and subsequent evaporation. In addition, the contact between the lipids and the aqueous solution of active agent is carried out at a temperature above the Tc.
Moreover, it is reported that, where liposomes are made without using organic solvents, other manipulations, which may result in formulations with certain unfavorable characteristics, are generally required. For example, U.S. Pat. App. Pub. No. 2008/0274172 describes methods of preparing liposomes containing at least two phospholipids without using organic solvents. However temperatures above the Tc were used to obtain stable liposomes from aqueous solutions containing inorganic salts.
Consequently, existing methods for preparing liposomes utilize organic solvents, protein, inorganic salts, and/or heat. Due to their toxicity and flammability, organic solvents are undesirable in the preparation of liposomes for pharmaceutical, cosmetic and other uses. Moreover, the use of organic solvents and proteins has negative repercussions in terms of production costs, safety, work hygiene and the environment. Similarly, the use of heat in the preparation of liposomes is undesirable in terms of production costs, safety, and the environment. The use of inorganic salts in the preparation of liposomes is undesirable as the introduction of inorganic salts increases the size of the liposome and/or results in a more turbid formulation. See e.g. Castile et al., International Journal of Pharmaceutics, 1999, vol. 188, issue 1, pp. 87-95. Thus, there is a need for a method for preparing liposomes without the use of undesirable agents and procedures.